Polyurethanes are widely used in various applications because of their good physical properties and the ability to prepare such resins in organic solvents, in bulk and, preferably, in aqueous medium. Much prior art exists dealing with various methods for producing aqueous polyurethane resins and polyurethane dispersions, but none appear to disclose a method of preparing water dispersible non-uniform, in particular core-shell polyurethanes.
Core-shell emulsion polymer particles are well known in the art but they deal primarily with radically polymerizable unsaturated monomers such as acrylates and vinyls. They are generally formed by a two-stage sequential emulsion polymerization process (see, for example, U.S. Pat. No. 6,479,147 (2002) and references cited therein). In the first stage, a monomer or a mixture of monomers are polymerized by a conventional emulsion polymerization technique forming a first-stage particle. In the second stage, a second mixture of polymerizable monomers and a polymerization catalyst are added to the first-stage emulsion to effect polymerization. In the second stage, the coating (“shell”) around the first-stage particle (“core”) is formed. If desired, multiple shells can be made over the same particle by sequential polymerizations. A similar approach is used to make polyurethane-acrylic hybrids as described in U.S. Pat. Nos. 4,644,030 and 5,137,961 only instead of the acrylic emulsion, a polyurethane dispersion is formed in the first stage.
U.S. Pat. No. 5,959,003 discloses that two diverse polyols are reacted with diisocyanates and two different amounts of hydrophilic group-containing, dihydric alcohols to obtain a hydrophobic prepolymer and a hydrophilic prepolymer respectively. The two prepolymers are then dispersed in water together but in separate streams (not mixed prior to dispersion) and purportedly core-shell aqueous polyurethane particles are thus claimed to be formed due to the different hydrophilicity of the two prepolymers. In fact as is demonstrated, true core-shell morphology is not obtained by the process disclosed in this patent.
U.S. Pat. No. 6,017,998 discloses a stable one-component aqueous polyurethane dispersion comprising the reaction product of (a) at least one water dispersible isocyanate-terminated polyurethane prepolymer (A); (b) at least one aqueous polyurethane dispersion (B) comprising active hydrogen atoms; and (c) water. It is stated that the reaction product is in the form of at least one of the structures selected from a grafted polymer, an interpenetrating network (IPN), a core/shell structure and mixtures thereof. The prepolymer (A) is a reaction product of (i) at least one polyisocyanate; (ii) at least one hydroxyl carboxylic acid; (iii) at least one polymeric polyol; and (iv) optionally, at least one sulfonated polyol. The aqueous polyurethane dispersion (B) is the reaction product of (i) at least one isocyanate terminated polyurethane prepolymer (B); (ii) at least one amine chain extender; (iii) alcoholamine chain terminator; and, optionally, (iv) water. The polyurethane prepolymer (B) is based on at least one different polyisocyanate, or at least one different polymeric polyol, or based on a different ratio of polyisocyanate/polyol as compared with those of polyurethane prepolymer (A). Said patent discloses that the polyurethane dispersion is prepared by dispersing the isocyanate-terminated polyurethane prepolymer (A) in the aqueous dispersion of polyurethane (B).
According to the process disclosed in the above-noted patent, one component is already made into a stable dispersion. In such a dispersion, particles are colloidally stable due to the double electric layer and/or steric hindrance, which prevent their flocculation, and the particles have inherent resistance to coalescence with other particles. The water creates a barrier between the prepolymer A and dispersion particles B. Therefore, the final product will be a rather heterogeneous mixture of different types of particles, such as (1) pure particles A, (2) pure particles B, and (3) hybrid particles A+B (if any). This is supported by Figure B of the cited patent, where the object of invention contains five separate populations of the particles different in their particle sizes.
The present invention overcomes the above-mentioned obstacle by intimately pre-mixing the two or more prepolymers before the dispersion step. Thus, more uniform particles are formed and the population of hybrid particles (containing prepolymers A and B) is greatly increased to at least above about 50% of the total weight of particles.